1. Field of the invention
The present invention relates to a method for in-bath smelting reduction of metal oxides, and more particularly to an in-bath smelting reduction method for production of iron and ferro-alloys, and an in-bath smelting reduction furnace.
2. Description of the prior art
In connection with iron melt reaction vessels such as the oxygen converter, various methods have been developed for enabling more efficient feeding of iron ore as a coolant, coke and other carbon materials as a heat source, and fluxes. In converter operation, however, it was for a long time the general practice to feed most materials in lump form, and where a powder material was to be supplied, to first form it into lumps and then to charge the lumps into the furnace from the top under their own weight.
On the other hand, recent years have seen vigorous efforts to develop ways for supplying iron ore, carbon materials and other materials to converters in powder form. JP-B-62-24486, for example, discloses a technique for using powder materials as a slag suppressing agent. According to the disclosed method, powder material is supplied through an opening of the converter side wall when the slag within the converter foams and tends to slop.
There have also been seen positive efforts to adopt in-bath smelting reduction of ore, a method in which ore, carbon material as both a reducing agent and a heat source, lime as a flux and the like are supplied into molten metal (hereinafter called the "hot metal") in a smelting furnace, and oxygen gas is blown onto the hot metal.
JP-A-61-199009, for example, discloses a method for supplying the materials in such in-bath smelting reduction, in which the ore, carbon materials and other materials are classified into lumps and powder by a comb, the lump materials of greater than a prescribed size are fed into the furnace from a material charging apparatus installed above the furnace, and the powder materials smaller than the prescribed size (hereinafter called "powder materials") are blown into the molten iron or the slag layer within the furnace by a powder supply apparatus.
For enabling efficient and economical supply of the great amount of heat that is consumed within the furnace in in-bath smelting reduction, numerous different attempts have been made toward raising the post combustion ratio so as to increase the amount of heat generated per unit amount of coal. The possibility for such an increase comes from the fact that while the amount of heat generated by oxidizing carbon to carbon monoxide is only about 2200 kcal/kg - C, that generated when carbon is oxidized to carbon dioxide is about 7800 kcal/kg - C, thus enabling generation of more heat per kg of carbon and Nm.sup.3 of oxygen. Moreover, as hydrogen burns only during post combustion, post combustion is indispensable for effective utilization of the combustion heat of hydrogen.
JP-A-62-280311 discloses a method of operating an in-bath smelting reduction furnace which aims at increasing the amount of heat generated from coal and improving heat efficiency by obtaining a high post combustion ratio and efficiently transferring the heat of the post combustion to the hot metal.
However, even if the post combustion ratio in the in-bath smelting reduction furnace is increased, insofar as fuel coal having a high volatile matter (hereinafter called VM) content of 25-40% is directly used in the furnace as fuel, the practical limit on the post combustion ratio becomes about 40-50%. This is because high VM-content fuel coal exhibits poor combustion characteristics in the in-bath smelting reduction furnace so that when the furnace is operated at a high post combustion ratio, the heat transfer efficiency becomes poor.
Thus when fuel coal is used, the heat transfer efficiency rapidly worsens during high post combustion operation. A problem in operation therefore arises that in the case where the post combustion ratio is increased in the in-bath smelting reduction furnace using fuel coal, the amount of heat generated per unit volume of the space in the furnace above the slag increases, with the result that the gas temperature rises sharply.
For example, if the post combustion ratio should be 50%, then even if operation should be conducted at the relatively good heat transfer efficiency of 80%, the gas temperature at the top of the furnace will become 1900.degree. C., which is much higher than the refractory at the top of the furnace can withstand. This is hardly conducive to economical metal production.
While there are a number of operational factors that affect the refractory wear rate, the most effective method for reducing wear is to reduce the temperature at the refractory hot face. As it is known that the refractory wear rate is increased when the portion of the refractory in contact with gas is exposed to highly oxidative gas, another effective way of protecting the refractory is to reduce the oxidation degree of the gas.
Combined reduction of the temperature and oxidation degree of the gas is, however, incompatible with reducing the unit consumption of coal by increasing the post combustion ratio and transferring a large amount of heat to the hot metal. By the conventional methods, therefore, it has been difficult to carry out operation at low unit consumption of coal while realizing long refractory service life.
On the other hand, pre-reduction of ore as the raw material is an effective method for reducing the unit consumption of coal. This is related to the fact that iron ore used for producing the hot metal is mainly hematite (Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4) and where C is consumed for reducing these iron oxides, the amount of reduction heat becomes huge, making it impossible to reduce the unit consumption of coal.
By pre-reducing the iron ore, however, the oxygen content of the iron oxides can be reduced, whereby the amount of carbon consumed for reduction can be lowered. At the same time, since the amount of reduction heat required when the iron oxides are reduced decreases, it becomes possible to reduce the amount of coal combustion heat. For these reasons, pre-reduction of the iron ore is a highly effective method of reducing the unit consumption of coal.
The gas generated from the in-bath smelting reduction furnace consists of H.sub.2 O, H.sub.2, CO, CO.sub.2 and N.sub.2, and in the in-bath smelting reduction of iron, effective utilization of this gas for pre-reduction of iron ore is highly preferable. A number of different methods for this purpose have been proposed.
In one method for efficient pre-reduction of iron ore, the reduction efficiency in the pre-reduction furnace is increased by cooling the gas to remove as water the H.sub.2 O which tends to hinder the ore reduction, and by removing carbonic acid in the gas in order to reduce amount of CO.sub.2, which also hinders reduction.
However, for removal of water and carbonic acid, it is necessary to cool the gas to room temperature and then to reheat it to 800.degree.-1000.degree. C., the temperature required for using it to pre-reduce iron ore.
By this method, the oxidation degree ((P.sub.CO.sbsb.2 +P.sub.H.sbsb.2 O)/(P.sub.CO.sbsb.2 +P.sub.CO +P.sub.H.sbsb.2 +P.sub.H.sbsb.2 O)) of the gas for the pre-reduction can be reduced to an appropriate ratio for ore reduction and the chemical equilibrium and reaction rate of the ore reduction can be improved so that it is possible to obtain a reduction ratio that is preferable for the in-bath smelting reduction.
However, this pre-reduction method proves to be uneconomical since it requires huge investment cost for such equipment as a waste heat boiler for cooling the gas generated by the smelting reduction furnace, a dust catcher, a water remover, a remover of carbonic acid, a gas holder for preventing supply-demand imbalances, a gas pressurizer and a heat exchanger for heating gas.
For avoiding the need for such large scale equipment and reducing investment costs, there has been developed an ore pre-reduction method wherein the gas generated from the smelting reduction furnace is not cooled to room temperature but only to the temperature appropriate for pre-reduction and the gas is supplied directly to the pre-reduction furnace and used for the pre-reduction of ore. This method is effective toward reducing investment costs since about the only equipment required is a recovery duct for the generated gas and a dust catcher.
With this method, however, the oxidation degree of the gas generated by the smelting reduction furnace is determined by the post combustion ratio in the in-bath smelting reduction furnace and therefore cannot be reduced.
With this method, if the operation is carried out at a low post combustion ratio in the in-bath smelting reduction furnace of around 20-30%, then as might be expected from the chemical equilibrium at a pre-reduction temperature of 800.degree.-1000.degree. C., the ore (Fe.sub.2 O.sub.3) is partially reduced to metallic iron and it is possible to obtain an ore pre-reduction ratio of about 40%.
At a pre-reduction ratio of 40% and a post combustion ratio of around 20% in the in-bath smelting reduction furnace, however, the unit consumption of coal required for operation comes to exceed 1100 kg/t, which is considerably greater than the 700-800 kg/t coal unit consumption of the coke oven - blast furnace method. It is thus impossible to produce hot metal economically using this method.
An improvement of the condition of heat balance in the in-bath smelting reduction furnace can be realized by an operating method in which the unit consumption of coal is reduced by increasing the post combustion ratio in the in-bath smelting reduction furnace so as to increase the amount of heat supplied to the hot metal per unit coal weight. However, the oxidation degree of the gas generated from the in-bath smelting reduction at a high post combustion ratio of 40-50% is such that adequate pre-reduction of the ore cannot be obtained if the gas is directly used for pre-reduction without removal of water or carbonic acid.
This is because if gas with such an oxidation degree should be used, at a reduction temperature in the range of 800.degree.-1000.degree. C. ordinarily used in pre-reduction furnaces, it would be possible, under conditions of chemical equilibrium, to realize reduction only to as far as Fe.sub.3 O.sub.4. Thus in the case of hematite ore (Fe.sub.2 O.sub.3), the pre-reduction ratio would be about 11% and in the case of iron sand and other magnetite ores, there would be no reduction whatsoever. What this means is that the reduction would be controlled by the chemical equilibrium determined by the composition of the gas after post combustion, making it impossible to obtain a high pre-reduction.
In an in-bath smelting reduction furnace that directly uses fuel coal, therefore, the pre-reduction ratio remains at a low level even if the post combustion ratio is improved to 40-50%, so that the unit consumption of coal cannot be sufficiently reduced and, at best, is only about 900 kg/T.
As was mentioned earlier, it is known that the post combustion ratio in the in-bath smelting reduction furnace is strongly influenced by the average VM content of the carbon materials.
Fuel coal, which is cheap and is produced in large quantities, normally has a VM content of 25-40%. Where it is attempted to produce hot metal economically using fuel coal, if the fuel coal is used in the in-bath smelting reduction furnace as it is, the post combustion ratio at which a relatively high heat efficiency is attained is 40-50%. That is to say, for supplying the heat required for smelting reduction of the ore, it is further necessary to reduce the unit consumption of coal by further simultaneously increasing the post combustion ratio and the heat transfer efficiency.
One way of doing this is by partially carbonizing the coal to lower its VM content. However, this leads to an increase in facility costs because the ordinary method of coal carbonization requires a special coal carbonization furnace, and there are also additional costs for the heat required for the carbonization.
Thus unless some method is devised for avoiding the need for a special carbonation furnace and for carrying out the carbonation efficiently, it is impossible to produce hot metal by using coal which has been partially and economically carbonized.
It is for this reason that there has been a desire to develop technology making it possible to reduce the unit consumption of coal by enabling both coal carbonization and reduction of the oxidation degree of the gas generated by the in-bath smelting reduction, and also enabling economical coal carbonation and gas reforming, without leading to an increase in running costs and investment cost.
In other words, for economical production of hot metal there is needed a method which is capable of reducing the oxidation degree of the gas generated by the smelting reduction furnace and also capable of partially carbonizing coal, with simple and inexpensive equipment.
When fuel coal is unloaded at the steelworks, it contains around 10-20% powder of a particle size of under 2 mm, owing partly to pulverization during transport and to powder produced during screening.
Unless this coal powder (slack) can be effectively utilized, the excess has to be processed into usable form or abandoned, which leads to increased costs for the processing equipment. For economical hot metal production it is therefore important to make effective use of this slack. If the slack is charged into the in-bath smelting reduction furnace from the top, however, the upflow of gas generated by the in-bath smelting reduction furnace entrains and carries away a part of the slack so that it cannot be efficiently utilized.
Regarding coal carbonization and gas reforming (a method for reducing the oxidation degree of gas), JP-A-62-283190, for example, discloses a method for gasifying coke using gas generated by an in-bath smelting reduction furnace or a coal gasifier. By this method, only highly gas permeable lump coke can be used as the carbon material for gasification.
This is because if coal containing VM should be used in this method, tar produced during carbonization would liquefy when the gas temperature decreased and the liquefied tar would obstruct the passage of gas. It is therefore difficult to carbonize coal including powder by this method.
It is known that powder ore is easier to mine than lump ore and that its quality can be improved by washing with water or floatation. Moreover, making good use of this powder ore is important for economical production of hot metal.
Also, if it should be possible to use powder ore for refractory cooling and the like, this would not cause any increase in cost or entail any loss of heat by feeding otherwise unnecessary substances into the furnace. This same principle can also be adopted in respect of powder fluxes such as lime.